This invention relates to improvements in valves for regulating gas pressure, particularly valves used to regulate the pressure of breathing gas in self-contained underwater breathing apparatus, commonly referred to by the acronym SCUBA. In such apparatus, air, or other breathable combination of gases, is stored under high pressure in one or more cylinders carried by the diver. It is necessary for the gas to be supplied to the diver at the same pressure as the surrounding water, which varies with depth, and at a rate which matches his respiratory requirements. These functions are performed by a pressure and flow regulating arrangement commonly referred to as a SCUBA Regulator or Demand Regulator. Similar regulators are also used in self-contained breathing apparatus for purposes other than diving, such as in firefighting, rescue and escape apparatus, which protect the wearer from irrespirable atmospheres. The present invention may be applied with advantage to any such regulator.
Depending upon the design of the cylinder(s) used to store the gas, the fully charged pressure may vary between about 2200 and 4500 psig and, as gas is consumed during use, the pressure may fall to as little as 200 psig at the end of the useful duration of the apparatus. Because of this considerable variation in the pressure of the stored gas, it is usual for SCUBA regulators to be composed of two distinct valve assemblies, commonly referred to as stages. The first stage, which connects to the outlet of the cylinder(s), is a pressure reducing valve, or pressure regulator, the purpose of which is to reduce the stored gas pressure to a substantially constant pressure, typically on the order of 130 psi. The reduced pressure is supplied, via an interconnecting flexible hose, to the second stage or demand valve, so called because it supplies gas to the diver on demand. It is important that the reduced pressure be maintained at a relatively constant level for the demand valve to operate properly.
There are several designs of pressure regulator in common use and, while they may vary considerably in construction details, all are required to perform the same basic functions. These pressure regulators utilize a spring loaded piston, diaphragm or bellows as a control mechanism to reduce the cylinder pressure. During a dive, the ambient pressure rises as the depth increases. At approximately 10 meters, the absolute ambient pressure is twice that at the surface, and the pressure increases an additional atmosphere for each additional 10 meters. If no means were provided to compensate for this increase in ambient pressure, the difference between the pressure regulator output pressure and the diver's lung pressure would significantly decrease as the diver descended, thus decreasing the maximum flow possible. Eventually the pressure regulator would not be able to supply sufficient air to the diver at even moderate work rates. Consequently, all pressure regulators have some provision for compensating for the change in ambient pressure throughout a dive.
The most common means of compensating for change in ambient pressure is to allow the surrounding water to freely enter the pressure regulator to transmit the ambient pressure to the control mechanism. This simple expedient has a number of disadvantages. First, moving seals are thus exposed to water which very often contains small abrasive particles in suspension The seals are normally lubricated with a synthetic grease to which the particles adhere. Causing abrasion to the sealing surfaces and to the seals themselves which results in leakage of gas past the seals and a general reduction in the useful life of the pressure regulator. The problem is compounded by the accumulation of salts dissolved in the water, being deposited as a solid crust in the sealing areas by evaporation of water remaining in the pressure regulator after use.
A further and potentially dangerous result of allowing water into the interior of the pressure regulator is that of the water freezing inside the pressure regulator during use and preventing movement of the control mechanisms. The rapid expansion of gas through the regulator, from high to low pressure, causes a considerable drop in temperature during use, particularly when the diver is breathing heavily and, in cold water, ice can form inside the pressure regulator causing it to totally fail.
Pressure regulators for use under adverse conditions and in cold water have to be protected from the ingress of water while still allowing ambient pressure to act on the control mechanisms. There are presently two commonly used methods to achieve this. The first and most common method is to fill the interior of the pressure regulator with a fluid with a freezing temperature lower than water such a glycol or, more usually, silicone grease. If the fluid is a liquid, it has to be sealed into the pressure regulator by means of a flexible diaphragm capable of transmitting water pressure to the fluid and hence to the piston head. A viscous grease is also used without a diaphragm but with very small fill ports to minimize loss of grease. However, as the control mechanism moves in use, the grease is displaced and can gradually become replaced with water. Both methods require complete filling of the pressure regulator without any voids and are messy and inconvenient, severely hampering routine maintenance.
An improved arrangement, described in U.S. Pat. No. 4,226,257, consists in sealing the pressure regulator, save for a small pressure relief valve in the form of a rubber mushroom having its outer face exposed to the surrounding water. A small orifice, or fine sintered metal plug, admits a very limited but continuous flow of gas from the outlet port area into the control mechanism. The flow of gas will cause pressure to rise to the level required to open the pressure relief valve, which will then vent the excess continually into the surrounding water. By this means, pressure inside the pressure regulator is maintained at a level very slightly above that of the surrounding water. The arrangement has the merit of keeping the pressure regulator clean and dry, and does not hinder dismantling and maintenance of the regulator. However, there are disadvantages to the arrangement. Firstly, the very fine orifice or sintered plug, which meters gas can become clogged by impurities or by improper handling and thus impair the controlled flow. Also, because flow is kept small so as to limit the loss of breathing gas, the system cannot compensate for rapid increases in depth and there are circumstances in which this limitation can be dangerous to the diver.